ISO 23777:2023

Pulps - Kraft liquor - Determination of hydrosulphide ion concentration using potentiometric titration

This document describes a procedure for the determination of sulphide, i.e. the hydrosulphide ion concentration, in white, oxidized white and green liquors, as well as in black liquor having a dry matter content up to 40 %. The determination also includes the sulphide part of any polysulphide present in the solution.

Pâtes — Liqueur Kraft — Détermination de la concentration en ions hydrosulfurés par titrage potentiométrique

General Information

• Status: Published
• Publication Date: 09-Mar-2023
• ICS: 85.040 - Pulps
• Technical Committee: ISO/TC 6 - Paper, board and pulps
• Drafting Committee: ISO/TC 6 - Paper, board and pulps
• Current Stage: 6060 - International Standard published
• Start Date: 10-Mar-2023
• Due Date: 20-Dec-2023
• Completion Date: 10-Mar-2023

Overview

ISO 23777:2023 - "Pulps - Kraft liquor - Determination of hydrosulphide ion concentration using potentiometric titration" - specifies a laboratory procedure to determine sulphide (hydrosulphide, HS−) concentration in kraft process liquors. The method applies to white, oxidized white, green and black liquors (black liquor up to 40% dry matter) and includes the sulphide fraction originating from any polysulphides present. The determination is based on potentiometric titration with silver nitrate, with pretreatment steps to dissolve polysulphides and clear interferences.

Key topics and requirements

• Scope and applicability: Suitable HS− ranges for white/green/black liquors (0.02–2.0 mol·L−1) and oxidized white liquors (0.5–50 mmol·L−1); sample volume adjusted accordingly.
• Principle: Potentiometric titration using AgNO3 to form Ag2S/AgSH precipitates; polysulphides are converted to HS− using alkaline sodium sulphite so each polysulphide contributes one sulphide equivalent.
• Reagents: Analytical-grade chemicals; oxygen-free water; 0.1 M AgNO3 (standardized against KCl); 1 M NaOH; alkaline sodium sulphite (0.5 M); ammonia (25%).
• Apparatus: Automatic potentiometric titrator (or manual titrator + pH meter), polished silver indicator electrode, reference electrode, calibrated pipettes.
• Sampling and pretreatment: Prevent oxidation (fill bottles fully); heat and react sample with alkaline sodium sulphite for polysulphide dissolution (different conditions for oxidized white liquor).
• Titration and calculation: Titrate to first inflection (~−650 mV) and calculate HS− concentration from AgNO3 consumption; results reported to two decimal places. Duplicate determinations and precision limits are provided (Annex A).
• Quality control: Electrode checks, appropriate dilution for samples >40% dry matter, and rejection/re-treatment rules if two inflection points indicate untreated polysulphide.

Applications and users

• Pulp and paper mills: Process control of kraft pulping and recausticizing operations; monitoring white and green liquor quality.
• Analytical laboratories: Routine laboratory method for QA/QC and compliance testing.
• Environmental and research labs: Measuring sulphide speciation in process effluents and research on sulfur balance in kraft recovery cycles.
• Plant chemists and laboratory analysts: Use this standard to ensure consistent, traceable hydrosulphide measurements and to interpret polysulphide contributions.

Related standards

• This ISO document contains no normative references. It was prepared by ISO/TC 6 (Paper, board and pulps); users may consult related ISO/TC 6 publications or national implementation guidance for complementary sampling and laboratory practices.

Keywords: ISO 23777:2023, kraft liquor, hydrosulphide ion, potentiometric titration, silver nitrate titration, polysulphide, white liquor, black liquor, sodium sulphite, pulp mill quality control.

Standards Content (Sample)

INTERNATIONAL ISO
STANDARD 23777
First edition
2023-03
Pulps — Kraft liquor — Determination
of hydrosulphide ion concentration
using potentiometric titration
Pâtes — Liqueur Kraft — Détermination de la concentration en ions
hydrosulfurés par titrage potentiométrique
Reference number
© ISO 2023
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may
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Published in Switzerland
ii
Contents Page
Foreword .iv
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Principle . 1
5 Reagents . 2
6 Apparatus . 3
7 Sampling and sample pretreatment.3
7.1 General . 3
7.2 White, green and black liquors . 3
7.3 Oxidized white liquors. 3
8 Procedure .4
8.1 General . 4
8.2 White, green and black liquors . 4
8.3 Oxidized white liquors. 4
8.4 Titration. 4
9 Calculation . 5
10 Report . 6
Annex A (informative) Precision data . 7
Bibliography . 8
iii
Foreword
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This document was prepared by Technical Committee ISO/TC 6, Paper, board and pulps.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www.iso.org/members.html.
iv
INTERNATIONAL STANDARD ISO 23777:2023(E)
Pulps — Kraft liquor — Determination of hydrosulphide
ion concentration using potentiometric titration
1 Scope
This document describes a procedure for the determination of sulphide, i.e. the hydrosulphide ion
concentration, in white, oxidized white and green liquors, as well as in black liquor having a dry matter
content up to 40 %. The determination also includes the sulphide part of any polysulphide present in
the solution.
2 Normative references
There are no normative references in this document.
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminology databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at https:// www .electropedia .org/
3.1
hydrosulphide ion concentration
concentration in white, black and green liquors or oxidized white liquor
2-
Note 1 to entry: Practically no S ions are present in white, oxidized white, green, and black liquors because of
hydrolysis according to the reaction:
2−− −
SH+→OHSO+ H
4 Principle
The procedure is based on potentiometric titration with silver nitrate solution according to
reaction (1) and (2):
−− +
HS ++OH 2Ag →+Ag SH O (1)
22()black ppt
−+
CH SA+→gAgCHS (2)
33 ()yellow ppt
Sulphite ions are added to the sample solution in order to dissolve any polysulphide ions that are
present according to reaction (3):
2−−2−−2 −
nSOS++SH On→+SO HS +OH (3)
3 n 22 3
Each polysulphide ion thus contributes one sulphide ion to the sulphide content. As the titration is
performed in alkaline solution, thiosulphate or sulphite ions do not interfere in the determination.
When sulphite ions are added to organic polysulphides, mercaptide ions are produced according to
reaction (4):
2−− −
CH SSCHS+→OCH SSO +CH S (4)
33 3 33 3
For white, green and black liquor, this document is applicable for hydrosulphide ion concentrations
from 0,02 mol/litre to 2 mol/litre, provided that the volume of the original sample taken to analysis
is selected accordingly. For oxidized white liquors the standard is applicable to hydrosulphide ion
concentrations from 0,5 mmol/litre to 50 mmol/litre, provided that the volume of the original sample
taken to analysis is selected accordingly.
Precision data are available in Annex A.
5 Reagents
All chemicals shall be of analytical grade.
5.1 Water, use oxygen-free water for the preparation of the solutions.
NOTE Oxygen-free water can be prepared from distilled water either by boiling the water for 15 min or by
displacing the oxygen with nitrogen gas.
5.2 Silver nitrate 0,1 M, dissolve 17,0 g of dry silver nitrate, AgNO , in a 1 000 ml volumetric flask.
Stir and fill up to the mark with water (5.1). Determine the concentration in the following way:
Weigh approximately 750 mg of dried potassium chloride, KCl, to an accuracy of 0,5 mg into a 100 ml
volumetric flask and fill up to the mark with water (5.1). With a precision pipette, take 5 ml for titration
in distilled water. Titrate with the silver nitrate solution to the first inflection point. From the silver
nitrate consumption, a ml, calculate the silver nitrate concentration in moles/litre to four decimal
places.
Calculate the silver nitrate concentration, [AgNO ], in mol/litre, according to Formula (5):
5*b
AgNO = (5)
[]
74,*5513 a*100
where
a is the silver nitrate consumption, in millilitres;
b is the amount of potassium chloride weighed, in milligrams;
74,551 3 is the relative molecular mass of KCl.
Store the silver nitrate solution in a dark glass bottle. Commercially available solutions of silver nitrate
may be used.
5.3 Sodium hydroxide solution 1 M, dissolve 40,0 g of NaOH in 1,0 litre of oxygen-free water (5.1).
5.4 Alkaline sodium sulphite solution approximately 0,5 M, dissolve 60,
...